1. Cleanup some **kwds passing instead of explicitly listing some random subset of arguments to cython().

2. Remove the import_test function. Instead, move the tests to doctests.

3. Make sure that all opened files are explicitly closed (typically using a with block).

4. In cython_import, restore sys.path after changing it.

5. Deprecate cython_create_local_so which is just calling cython() with specific keyword arguments.

6. Use the .pyx extension for all temporary files.

7. Use the restore_cwd context.

Component: cython

Author: Jeroen Demeyer

Branch/Commit: af2c010

Reviewer: Julian Rüth

Issue created by migration from https://trac.sagemath.org/ticket/24722